## Lesson 12 pg323 Fluid dynamics/Blood Flow

vgp1993
Posts: 18
Joined: Wed Dec 20, 2017 12:14 pm

### Lesson 12 pg323 Fluid dynamics/Blood Flow

I was wondering if someone could clear this up for me-

I am sort of confused about how we are saying that the structures most likely to experience turbulent flow are short, curved, branched arteries, but then we say the dominant contributors of flow resistance are the arterioles. Why are arterioles not the most likely to experience turbulent flow if flow resistance comes from the rapid narrowing of vessels?

Also, is pressure lower in the capillaries than in veins? The system of capillaries has the largest total radius, so Poiseuille's law would say that the resistance and pressure are low, but the velocity is slower there than in veins, so that would indicate a higher pressure. Am I incorrect in my understanding about one of these relationships?

Thanks!
NS_Tutor_Andrew
Posts: 520
Joined: Mon May 23, 2016 1:47 pm

### Re: Lesson 12 pg323 Fluid dynamics/Blood Flow

Hi vpg1993,

Re: your first question, I think the simplest answer (w/ the qualification that I'm not personally a qualified expert in hemodynamics) is that turbulence and resistance are generally distinct phenomena. The fact that turbulence is uncommon in the body but can occur in pathological conditions and is most likely at junctions/intersections is reasonably well-documented (see these lecture notes, for instance). Here's one of the foundational articles documenting the observation that bifurcations make turbulence most likely (i.e. reducing the critical value of Re), and recent research seems to suggest that non-Newtonian behavior is relevant. This of course goes beyond the scope of the MCAT, for which all you are likely to need is a general awareness of the Reynolds number + the fact that messing with flow by doing things like introducing constrictions/bifurcations makes turbulence more likely. Peripheral vascular resistance is another beast entirely, so these concepts ultimately aren't linked that closely.

As far as I know, pressure is lower in veins than in capillaries: see here for instance, and this seems to be pretty consistent across sources.

Hope this helps resolve your questions, & best of luck as you study!
Andrew D.
Content Manager, Next Step Test Prep.
vgp1993
Posts: 18
Joined: Wed Dec 20, 2017 12:14 pm

Great, thanks!